The present invention relates to a rotation rate measuring instrument utilizing the Sagnac effect to determine the rotation rate. An instrument of this kind is described in German Patent Application No. P 31 36 688.
In the instrument described there, a light beam is split into two component beams which travel in opposite directions around an optical waveguide forming a closed light path. To achieve high measurement accuracy, it is desirable to operate the optical-to-electrical transducer at that point of its characteristic where a small change in the intensity of the light directed to the transducer causes a large change in the transducer's output signal. On the other hand, it is desirable to have an alternating-voltage signal for evaluation purposes. Both can be achieved by modulating the two component beams in a suitable manner. This will be illustrated by an example: For the modulators, Bragg cells are used. A Bragg cell is placed in front of each end of an optical waveguide, and the frequencies of the drive signals for the two Bragg cells are changed periodically. The modulating frequency is the frequency at which switching occurs between the different frequencies of the drive signals. The phase shift between the two component beams emerging from the optical waveguide depends on the frequencies of the drive signals for the two Bragg cells. These frequencies are so chosen that the phase difference between the two component beams is (2.nu.+1).multidot..pi./2 or (2.nu.+3).multidot..nu./2, where .nu. may be any positive or negative integer. The frequencies of the drive signals are changed at the modulating frequency f.sub.S so as to periodically produce one phase difference or the other.
When the measuring instrument is at rest, the output signal of the optical-to-electrical transducer to which the two component beams are directed after emerging from the optical waveguide is the same for both phase differences between the component beams. When the measuring instrument rotates, the phase difference caused by the Sagnac effect adds to the above-mentioned phase difference, and the output of the optical-to-electrical transducer provides an alternating-voltage signal having the frequency f.sub.S of the modulating signal. By controlling the frequencies of one or both drive signals, an additional phase difference is produced between the two component beams which is chosen so as to compensate for the Sagnac phase difference, i.e. the phase differences (2.nu.+1).multidot..pi./2 and (2.nu.+3).multidot..pi./2 are obtained again. From the control signal, the rotation rate .OMEGA. is determined.
If the compensation for the Sagnac phase difference .PHI..sub.S is performed at large values of .nu., the equation by which the rotation rate .OMEGA. is determined contains a large additive term which depends on .nu.. The quantities of this term (length and refractive index of the optical waveguide) may drift and, consequently, cause measuring errors. Therefore, calibrations are performed in the prior art instrument at fixed time intervals. For the calibration, the frequencies of the drive signals for the Bragg cells are chosen so that a phase difference of 2.pi. or an integral multiple thereof exists between the two component beams emerging from the optical waveguide.